1. Field of the Invention
The present invention relates to a process and apparatus for producing high purity magnesium oxide fine particles by means of a gas phase oxidizing reaction.
2. Description of the Related Art
It is well known that fine particles of varioss metal oxides, for example, magnesium oxide and calcium oxide, exhibit excellent heat resistance and electrical insulating property and, therefore, are highly useful as ceramic materials, catalysts, pigments, or fillers in a wide range of industries. Especially, it was recently discovered that very fine metal oxide particles having a very small size of 0.1 .mu.m or less exhibit various unique properties different from those of coarse metal oxide particles. For example, very fine metal oxide particles exhibit excellent chemical reactivity due to their large total surface area and the high surface energy of the particles. Also, very fine metal oxide particles exhibit different magnetic and optical properties from those of usual metal oxide particles in the form of a bulk due to the very small volume of the individual particles.
The above-mentioned specific properties open up new fields of application for very fine metal oxide particles, for example, starting materials for catalysts, sintered materials, porous materials, sensor materials, magnetic materials, and pigments.
In particular, magnesium oxide fine particles are useful as starting materials of sintered materials and as sensor materials. Accordingly, there is a strong demand for the provision of high purity magnesium oxide fine particles.
It is known that the fine metal oxide particles can be produced by various methods including liquid phase reaction methods and gas phase reaction methods.
Particularly, in the gas phase reaction method, it is believed that very fine metal oxide particles can be produced by carrying out the metal oxide-forming reaction under appropriate conditions at a high efficiency, because in this method the resultant fine metal oxide particles do not easily agglomerate, the formation of secondary agglomerates is very small, and the reaction conditions can be easily decided.
Accordingly, there are various approaches to new gas phase reaction methods for producing high purity magnesium oxide particles.
The gas phase reaction methods can be classified into a first method, wherein metal vapor is brought into contact with an oxygen-containing gas at a temperature at which the metal vapor can be oxidized into fine metal oxide particles, and a second method, wherein metal oxide particles are produced in a combustion flame generated by the combustion of a corresponding metal substance which is capable of being oxidized.
In the first gas phase reaction method, for example, metallic magnesium is heated within an inert gas atmosphere to generate magnesium vapor, the magnesium vapor is allowed to flow into an oxidizing region, and a flow of a molecular oxygen-containing gas is introduced into the oxidizing region countercurrently to the flow of the magnesium vapor, to allow the magnesium vapor to come into contact and react with the molecular oxygen-containing gas. This method for producing fine magnesium oxide particles having a high purity is disclosed in Czechoslovakian Patent No. 139,208.
Also, Takanori Watari, Kazumi Nakayoshi and Akio Kato, Journal of Japanese Chemical Society, No. 6, pages 1075 to 1076 (1984), disclose a method for producing fine magnesium oxide particles in which metallic magnesium is heated and the resultant magnesium vapor is introduced together with argon gas into a reactor and is mixed with an oxygen (O.sub.2) - nitrogen (N.sub.2) mixture gas.
The above specifically mentioned methods are, however, disadvantageous in that, when magnesium vapor is fed into an oxidizing region through a front open end of a nozzle, a portion of the fed magnesium vapor is oxidized around the front open end of the nozzle and the resultant magnesium oxide particles deposit on and block the front open end of the nozzle. Because of this phenomenon, the oxidizing operation can not be carried out continuously over a long period of time. That is, the oxidizing operation must be interrupted to remove the deposits of magnesium particles from the front open end of the nozzle. Another disadvantage of the above-mentioned conventional methods is that, since the oxidizing operation is carried out at a high temperature of 800.degree. C. to 1600.degree. C., the deposited magnesium oxide is sintered on the front open end of the nozzle. This phenomenon results in a significant decrease in the yield of resultant non-sintered magnesium oxide particles.
Also, in the conventional methods, in order to produce very fine magnesium oxide particles having a very small size, it is usually necessary to dilute the magnesium vapor with a large amount of an inert gas and then bring the diluted magnesium vapor-containing gas into contact with a molecular oxygen-containing gas. The magnesium vapor is generated by melting and then vaporizing metal magnesium at an elevated temperature, and is then diluted by the inert gas in a diluting region. If the inert gas contains impurities, for example, oxygen and nitrogen, which are reactive with the magnesium vapor, the impurities will react with the magnesium vapor and the resultant magnesium compounds be deposited in the diluting region. Therefore, it is necessary to remove the reactive impurities from the inert gas. This necessity incurs a very complicated and expensive production process and apparatus, and a very high production cost of the resultant magnesium oxide fine particles.